[OSM-talk] Islands

Sat Apr 14 11:16:14 BST 2007

On 4/14/07, Artem Pavlenko <artem at mapnik.org> wrote:
>
>
>
> On 14 Apr 2007, at 10:47, 80n wrote:
>
> David
> I think there are a couple of problems with closing off the ends of the
> river sections.
>
> Firstly, purists would argue that a segment crossing the river is an
> artifice that does not represent anything in the real world.
>
> Second, you would need two segments, or the segment needs to belong to two
> ways (the upstream way and the downstream way).  If it belongs to two ways
> then for one of them it is pointing in the wrong direction. (segment sorting
> will fix that, but that requires running frollo before Osmarender).
>
> I've contemplated doing this myself several times and always managed to
> persuade myself not to.
>
> 80n
>
>
> 80n,
> We don't really need all this crossing segments.
> Please, could you explain to me what's wrong with representing features
> like 'river with islands' as proper polygons (one exterior and n-interior
> rings) ? If you want just a small part of a bigger polygon for rendering use
> _clipping_.
>

Artem
That's what I said wasn't it?  There should not be segments crossing the
rivers because:

"Firstly, purists would argue that a segment crossing the river is an
artifice that does not represent anything in the real world.

Second, you would need two segments, or the segment needs to belong to two
ways (the upstream way and the downstream way).  If it belongs to two ways
then for one of them it is pointing in the wrong direction. (segment sorting
will fix that, but that requires running frollo before Osmarender)."

80n

Cheers,
> Artem
>
>
>
>
>
> BTW I noticed an error on the Baghdad map.  The al-Sarafiya bridge is
> still shown, but it was blown up last week ;)
>
> On 4/14/07, David Groom <reviews at pacific-rim.net> wrote:
> >
> >  If the ends of the riverbank were closed off with segments so the
> > riverbank formed a complete polygon, would the method outlined below by
> > 80n be enough to render rivers and islands correctly in the mapnik layer?
> >
> > David
> >
> > ----- Original Message -----
> >  *From:* 80n <80n80n at gmail.com>
> > *To:* David Groom <reviews at pacific-rim.net>
> > *Cc:* talk at openstreetmap.org
> > *Sent:* Saturday, April 14, 2007 9:21 AM
> > *Subject:* Re: [OSM-talk] Islands
> >
> > One method that currently works is to tag islands as part of the same
> > way as the riverbanks, but make them go anti-clockwise.
> >
> > So you have one way that goes up the left bank of the river for, say,
> > 1km then goes down the right bank for for the same distance.  If the two
> > riverbanks are imagined to be part of a closed way then the segments should
> > point in a clockwise direction. Then all of the islands within that 1km
> > section are also made part of that same way, but tagged in an anti-clockwise
> > direction.
> >
> > This seems to work pretty well, even when cut at arbitrary points by a
> > bbox, and is understood by SVG so renders with Osmarender.  The same
> > principle also workd for islands in lakes and any other situation where
> > something has a "hole" in it.
> >
> > There are some examples along the River Thames:
> >
> > Simple example: http://www.informationfreeway.org/?lat=6698535.88756&lon=-57979.86205&zoom=16&layers=B000
> >
> > Complex example: http://www.informationfreeway.org/?lat=6688981.2611&lon=-51110.08562&zoom=16&layers=B000
> >
> >
> > 80n
> >
> > On 4/14/07, David Groom <reviews at pacific-rim.net> wrote:
> > >
> > >  I don't know :)
> > >
> > > I've been meaning to update the large rivers page [1] to get debate
> > > going on the whole large rivers  / lakes idea, which would include a
> > > discussion on how to render islands.
> > >
> > > Straying away from your initial question, and looking at large rivers,
> > > Osmarender currently renders large rivers, but the Mapnik layer does not.
> > >
> > > As I understand it Mapnik needs a proper polygon to generate the fill
> > > and so does not generate a filled wide river from the proposed rendering on
> > > the large rivers [1] page. Own its own this is fairly easy to
> > > implement.
> > >
> > > Moving now to your question about islands :
> > >
> > > It is, as far as I can see, fairly difficult to intuitively split a
> > > river with islands in it into polygons (particularly where there are many
> > > islands close to each other) , where each polygon would be a continuous way
> > > and there is not a complicated system of segments going between the
> > > riverbank and each island.
> > >
> > > I'd like to see a modification of my proposal on the large rivers page
> > > [1] where riverbanks would be drawn as a polygon with a tag something like
> > > waterway = riverbank, and islands in these rivers would be drawn as
> > > waterway = riverbank; island = yes.  Given this suggested rendering, island
> > > in lakes could be tagged exactly the same as islands in rivers and they
> > > would still render correctly.
> > >
> > > David
> > >
> > >
> > > [1]  http://wiki.openstreetmap.org/index.php/Proposed_features/Large_rivers
> > >
> > >
> > >
> > >
> > >  ----- Original Message -----
> > >  *From:* lewispusey <lewispusey at earthlink.net>
> > > *To:* talk at openstreetmap.org
> > > *Sent:* Friday, April 13, 2007 5:08 PM
> > > *Subject:* [OSM-talk] Islands
> > >
> > > What is the current best way to draw and tag islands in rivers and
> > > lakes?
> > > Lewis
> > >
> > > ------------------------------
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> Artem Pavlenko
> http://mapnik.org
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